1. Field of the Invention
The present invention relates to an optical device, an optical deflection device, and an optical modulation device which include a phase modulation element having: an optical waveguide layer which is formed of an electro-optic material and to which a laser light beam is inputted; and electrodes through which a voltage is applied to the optical waveguide layer to form a refractive index modulation region of the laser light beam.
2. Description of the Related Art
Optical devices, such as an optical deflection device, e.g., an optical beam scanner configured to perform scan with a laser beam (i.e., a laser light beam) and an optical modulation device configured to adjust the intensity of a laser beam, are used in a variety of fields, including a laser printer, laser processing, a display, measurement, optical communication, and the like.
Generally, the optical beam scanner includes a lasing device as a light source and an optical deflection element configured to perform scan with a laser beam. If necessary, the optical beam scanner may further include: a light coupling optical system configured to shape an optical beam (i.e., a laser beam) emitted by the laser light source into a beam having a shape suitable for an optical beam deflection element (i.e., an optical deflection element); and/or an output optical system configured to, for example, shape the profile of an optical beam outputted from the optical beam deflection element or to increase or decrease the deflection angle of the optical beam given by the optical beam deflection element.
Conventionally, general types of the optical beam deflector (i.e., optical beam deflection element) include not only one for performing optical scanning through control of the reflection angle of an optical beam, such as a rotating polygon mirror, a galvanometer mirror, and an MEMS mirror, but also one that utilizes a change in the refractive index of a material, the change being caused by an acousto-optical (AO) effect or an electro-optical (EO) effect.
Among these, the EO effect offers a very high response speed in principle, and therefore an optical beam deflection element formed of an electro-optic material is useful in high-speed beam scanning. Further, since the scan angle can be controlled according to the voltage applied to the EO material, a desired deflection angle can be instantly given to the optical beam.
An element formed of an electro-optic material is used in various fields (e.g., Japanese Patent No. 3704553 as well as Japanese Patent Application Publication Nos. 2001-337302, Hei 9-5797, and Hei 7-244307). However, optical damage, i.e., photorefraction is generally known as a problem caused when such an element is used as an optical waveguide layer by inputting a laser light beam into the element. Photorefraction is a phenomenon in which, when a high energy optical beam is applied to a material, the shape of the optical beam is largely distorted. Photorefraction is thought to be caused when free electrons, generated inside an electro-optical material by optical energy excitation, drift to form a random internal electric field inside the electro-optic material, or when the internal electric field formed by the free electrons causes an irregular change in the refractive index inside the electro-optic material by the electro-optic effect.
Photorefraction can be noticeably observed in an optical device in which an element formed of an electro-optic material is used as an optical deflector (i.e., optical beam deflector). The reason for this is as follows. The wavelength and the light intensity of a light source of an optical device vary depending on the intended use of the optical device, and a laser light source with a high intensity from several hundreds of mW to 1 W or higher is selected particularly for an optical device in which a laser beam is inputted to an optical deflector and which is used for laser processing or distance measurement with a laser radar or for laser projection.
To solve the problem of photorefraction, attempts have been made to optimize the composition of an electro-optic material. For example, lithium niobate and the like are known as a general electro-optic material or non-linear optical material. These materials have poor resistance to photorefraction. Accordingly, when used for a wavelength conversion element, these materials are doped with a metal material such as magnesium or iron in order, for example, to be able to support output of a short-wavelength laser beam such as a green laser. As described, it is known to increase the resistance of a material to photorefraction by doping the material with an appropriate amount of magnesium or the like to increase the photoconductivity of the material and thereby to decrease the internal electric field formed by photoexcited carriers. Another countermeasure against photorefraction proposed is to apply an ultraviolet light beam to the wavelength conversion device (see, for example, Japanese Patent No. 3704553).
Accordingly, also in a case of an optical deflection element used in the optical beam scanner, if the optical deflection element is formed of an electro-optic material, it is effective to use an electro-optic material excellent in resistance to photorefraction, depending on the light intensity of the scanning laser beam. This is true not only for an optical deflection device such as an optical beam scanner, but also for an optical modulation device.
However, in an optical deflector or optical modulator in which magnesium-doped lithium niobate is used as an electro-optic material for an optical waveguide layer, when a refractive index modulation region of a laser light beam is formed by application of a voltage to the optical waveguide layer, the profile of an optical beam propagating inside the optical waveguide layer has been observed to be largely distorted. This problem of beam shape distortion is especially noticeable in DC voltage operation. This is because, when a voltage is continued to be applied with always the same polarity, drifting of carriers by an external electric field are promoted to form a local electric field distribution in the refractive index modulation region.
In an optical deflector, this beam distortion greatly decreases performance such as the number of resolvable spots. In an optical modulator, the beam distortion decreases an extinction ratio.
In view of the above problems, in an optical device including a phase modulation element having: an optical waveguide layer which is formed of an electro-optic material and into which a laser light beam is inputted; and electrodes through which a voltage is applied to the optical waveguide layer to form a refractive index modulation region for the laser light beam in the optical waveguide layer, it is necessary to suppress the beam distortion of the laser beam caused in the refractive index modulation region formed in the optical waveguide layer when a voltage is applied to the optical waveguide layer through the electrodes, in order to output a beam having a favorable profile from the phase modulation element.
Further, in the above-described configuration of applying an ultraviolet light beam, a voltage is not applied to the wavelength conversion device. For this reason, this configuration may not be effective as a countermeasure against photorefraction caused upon application of a voltage. Moreover, as a configuration in which beam distortion can be suppressed even when a voltage is applied, the applicant has previously proposed a configuration of application of a pump light beam having a shorter wavelength than an optical beam (Japanese Patent Application No. 2011-199247). The pump light beam should desirably be applied as efficiently as possible.